The present invention relates to a screw compression apparatus and an operation control method thereof, particularly to a preferred screw compression apparatus and an operation control method thereof in which an inverter drive motor is used to control a capacity of a compressor body, the compressor body includes a pair of screw rotors, and the rotors are synchronously rotated in a non-contact manner to compress air and other operation gases.
As a compressor body for use in a screw compression apparatus, there are known an oil-free screw compressor, provided with a pair of screw rotors connected to each other via a timing gear, for synchronously rotating the pair of screw rotors in a non-contact and oil-free state, and an oil-cooled screw compressor for supplying oil to the pair of screw rotors, which mesh with each other, to rotate the rotors.
An example in which the oil-free screw compressor is used is disclosed in Japanese Patent Unexamined Publication No. 06-18584. In the compressor disclosed in the publication, a suction throttle valve is provided in a suction air passage of the compressor, and an air release valve for releasing compressed air from an air conduit on a suction side of a check valve is disposed midway in a compressor discharge air passage on an upstream side from a check valve. Moreover, during full-load operation, the suction throttle valve is opened and the air release valve is closed. Furthermore, a discharge pressure rises as an amount of air usage on a load side decreases. When a pressure detector detects an upper-limit pressure, the suction throttle valve is closed and the air release valve is opened.
On the other hand, another example is disclosed in Japanese Patent Unexamined Publication No. 09-287580 in which the oil-cooled screw compressor is operated by using an inverter drive motor. In this publication, an inverter is used to control a revolutional speed of the compressor in a compressor operating range in which the amount of the air consumption ranges from about 30% to 100% with respect to a specified discharge air amount as a discharge air amount in a rated output. Moreover, when the amount of the air usage is 30% or less of the specified discharge air amount and the discharge pressure (the pressure on the delivery side of the check valve) reaches a set pressure, the screw compressor continues to be operated at a set lower-limit rotation speed in the revolution number control. Furthermore, the suction throttle valve is closed to decrease the discharge pressure, and change-over to an unload operation is performed.
The aforementioned conventional oil-free screw compressor has an advantage that no oil is mixed into operation gases such as air, but no inverter is used and it is therefore difficult to arbitrarily adjust the rotation speed of the compressor.
On the other hand, in the aforementioned conventional oil-cooled screw compressor, since the inverter is used, the compressor rotation speed can be adjusted. However, since a lubricant oil or a cooling oil is mixed in the operation gas, it is necessary to separate the mixed oil after compression.
To solve the problem, it is considered that by applying the inverter used for the oil-cooled screw compressor to the oil-free compressor, cleaning of the operation gas and variable speed operation of the compressor can both be realized. However, even when the inverter is simply employed to keep the discharge pressure of the oil-free screw compressor to be a specified pressure, a ratio of an internal air leak amount to a swept air amount increases in a compressor low rotation speed area. As a result, there is a possibility that air having leaked to the upstream side is compressed again in a compressor compression chamber. When such phenomenon occurs, a compressed air temperature rises, and at a certain rotation speed or less, it becomes difficult to operate the compressor.
Moreover, for the compressor operated at the specified pressure, since an allowance between the upper limit value of the discharge pressure at which the compressor reaches its critical temperature, and the specified pressure becomes very small, there is a possibility that the upper-limit pressure is exceeded when controlling the rotation speed.
An object of the present invention is to provide a screw compression apparatus in which power consumption is reduced when changing a low load operation to an unload operation from a low load operation.
To attain the aforementioned object, according to a first aspect of the present invention, there is provided a screw compression apparatus comprising: a screw compressor including a pair of female and male rotors; a motor, controlled by an inverter, for driving the compressor; pressure detecting means for detecting a pressure of an operation gas discharged from the screw compressor (a pressure on a delivery side of a check valve); and an air release valve for performing a control to release the operation gas compressed by the compressor (the gas on a suction side of the check valve) to the atmosphere. The apparatus further comprises control means for controlling the motor and the air release valve. The control means controls rotation frequency of the motor by the inverter at an operation point at which a ratio of an air consumption on a user side to a specified compressor discharge amount is larger than a predetermined set value, holds the rotation frequency of the motor at a constant value when the ratio indicates the set value or less, controls the air release valve to release the operation gas (the gas on the suction side of the check valve) to the atmosphere after the pressure detected by the pressure detecting means reaches a set upper-limit pressure, and further lowers the rotation frequency of the motor after the pressure of the operation gas (the pressure on the delivery side of the check valve) reaches the upper-limit pressure.
In this aspect, when the operation gas pressure (the pressure on the delivery side of the check valve) reaches the set upper-limit pressure, the control means controls the air release valve and motor to release the operation gas (the gas on the suction side of the check valve) to the atmosphere via the air release valve and to lower the rotation frequency of the motor. There is provided a suction throttle valve for controlling an operation gas amount sucked by the compressor. When the amount of the air consumption indicates the set value or less of the air consumption ratio, the control means closes the suction throttle valve and releases the operation gas to the atmosphere via the air release valve, and subsequently controls the motor to further lower the rotation frequency of the motor. Alternatively, another air release valve may be provided in parallel to the air release valve.
To attain the aforementioned object, according to a second aspect of the present invention, there is provided an operation control method of a screw compression apparatus comprising: a screw compressor driven by a motor having an inverter; and pressure detecting means for detecting a pressure of an operation gas discharged from the compressor, the method comprising the steps of: controlling rotation frequency of the motor by the inverter when an air consumption on a demand side is larger than a predetermined set value; holding the rotation frequency of the motor at a constant value when the amount of the air consumption indicates the set value or less; controlling the air release valve to release the operation gas to the atmosphere when the pressure detected by the pressure detecting means reaches a set upper-limit pressure in this state; and further lowering the rotation frequency of the motor during or after air release.
Moreover, in this aspect, when the pressure of the operation gas indicates the set upper-limit pressure, the operation gas is released to the atmosphere via the air release valve and the rotation frequency of the motor is lowered. A suction throttle valve for controlling an operation gas amount sucked by the compressor is provided. When the amount of the air consumption indicates the set value or less, the suction throttle valve is closed and the operation gas is released to the atmosphere via the air release valve, and subsequently the rotation frequency of the motor may further be lowered.
To attain the aforementioned object, according to a third aspect of the present invention, there is provided an operation control method of a screw compression apparatus comprising: a screw compressor driven by a motor having an inverter; and pressure detecting means for detecting a pressure of an operation gas discharged from the compressor, the method comprising the steps of: holding a rotation frequency of the motor at a first rotation frequency when an amount of an air consumption indicates a predetermined set value or less; and controlling the rotation frequency of the motor to provide a second rotation frequency thereof lower than the first rotation frequency after the pressure detected by the pressure detecting means reaches a set upper-limit pressure in this state.
In this aspect, when the motor is held at the first rotation frequency and the pressure detected by the pressure detecting means reaches the set upper-limit pressure, the operation gas compressed by the compressor is released to the atmosphere. When the motor is operated at the second rotation frequency, before the amount of the air consumption returns to the set value, the motor is operated at a frequency higher than the second rotation frequency. When the motor is operated at the second rotation frequency and the operation gas is released to the atmosphere, the motor is decelerated to provide a lower-limit frequency. This state is held until the air consumption reaches the set value. Subsequently, when the set value is obtained, the air release may be stopped after accelerating the motor to obtain the first rotation frequency.